Metallic rollers are used in heat treatment furnaces for the heat treatment of metallurgical products or ceramic products. Typically the rollers are used in roller hearth furnaces for the heat treatment of carbon steel, stainless steel and nickel based alloy products. In the roller hearth furnace, an object to be heat treated is transported through the furnace by means of a plurality of rollers. Rollers are often made from centrifugally cast steel products because of the high strength of such a product and the comparatively low cost of the final product (including material and manufacturing costs). In this case, the roller consists of several parts which are produced separately and subsequently welded together in order to manufacture the roller.
One example of a previously known material for rollers in roller hearth furnaces is an austenitic nickel-chromium alloy comprising 23-30% Cr, 8-11% Fe, 1.8-2.4% Al, 0.01-0.15% Y, 0.01-1.0% Ti, 0.01-1.0% Nb and 0.01-0.2% Zr, as disclosed in U.S. Pat. No. 5,980,821 A. Another example of a previously known material for rollers is a nickel-chromium alloy comprising 55-65% Ni, 19-28% Cr, 0.75-2% Al, 0.2-1% Ti, 0.035-0.1% N, up to 0.1% C, up to 1% each of Si, Mo, Mn and Nb, up to 0.1% B and balance Fe, as disclosed in EP 0 251 295 A2. Yet another example of a previously known material for rollers is a cast nickel-chromium alloy comprising 15-40% Cr, 0.5-13% Fe, 1.5-7% Al, 0.01-0.4% Zr and 0.019-0.089% Y, as disclosed in WO 2004/067788 A1.
Conventional centrifugally cast rollers of Ni—Cr alloys often suffer from insufficient oxidation resistance due to spallation of the surface oxide. Furthermore, there is a risk of formation of surface defects, such as precipitation of hard particles of for example carbides, during use at high temperatures. Therefore, such rollers are often coated with a suitable coating material to prolong the service life. However, this type of roller still typically requires inspection every six months and reconditioning once a year due to the risks of surface defects or spallation of the coating. The total life time of this type of rollers is often in the range of two to three years. Re-conditioning means that the roller has to be removed from the furnace and machined, usually by turning, in order to accomplish the desired surface. In the case of these coated rollers, the roller also has to be re-coated. Reconditioning is a time consuming and costly process, especially since the furnace has to be shut down and the roller removed from the furnace for re-conditioning. Hence, the need for reconditioning reduces the productivity of the roller hearth furnace.
It is therefore an object of the present invention to find a suitable material to be used for rollers, intended for use in roller hearth furnaces for transportation of an object to be subjected to a heat treatment, which minimizes the need for reconditioning of the roller and thereby minimizes the productivity loss of the roller hearth furnace.